Cascade development process with two-component developer



Nov. 22, 1966 E. H. LEHMANN 3,287,150

CASCADE DEVELOPMENT PROCESS WITH TWO-COMPONENT DEVELOPER Original Filed July 18, 1962 INVENTOR. ERNEST H. LEHMANN ATTORNE) United States Patent 3,287,150 CASCADE DEVELOPMENT PROCESS WITH TWO-COMPONENT DEVELOPER Ernest H. Lehmanu, Rochester, N.Y., assignor to Xerox gorfioration, Rochester, N.Y., a corporation of New or Continuation of application Ser. No. 210,738, July 18, 1962. This application Feb. 10, 1965, Ser. No. 433,542 9 Claims. (Cl. 11717.5)

This application is a continuation of application Serial No. 210,738 filed July 18, 1962 which is a continuation in part of application Serial No. 784,706 filed January 2, 1959.

My invention relates to xeropraphy and more particularly to an improved process and means for applying electroscopic developer material to a surface bearing an electrostatic image.

In xerography it is usual to form an electrostatic image on a surface. One method of doing this is to charge a photoconductive insulating surface and then dissipate the charge selectively by exposure to a pattern of activating radiation as set forth, for instance, in US. 2,297,691 to C. F. Carlson. Other means of forming electrostatic images are set forth in US. 2,647,464 to I. P. Ebert. Whether formed by these means or any other, the resulting electrostatic charge pattern is conventionally utilized by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic image. Alternatively the electrostatic charge pattern may be transferred to an insulating film as disclosed, for example in US. 2,825,814 to L. E. Walkup and the electroscopic particles deposited thereon to form the visible image. In any case this visible image in turn may be transferred to a second surface to form a xerographic print or may be fixed directly to the photoconductive surface.

The usual process of applying the developer to the electrostatic image is set forth in US. 2,618,552 to E. N. Wise and involves the use of a finely-divided colored material called a toner deposited on a slightly more coarsely divided material called a carrier. This two-component developer is cascaded across the electrostatic image areas. The toner and carrier being rubbed against each other while cascading, impart an electrostatic charge to each other by triboelectric charging. To produce a positive of the electrostatic image, a toner and carrier are. selected such that the toner will be charged to a polarity opposite to that of the electrostatic image, the carrier being charged to the same polarity as the electrostatic image. When a carrier particle, bearing on its surface oppositely charged particles of toner, crosses an area on the image surface having an electrostatic charge, the charge on the surface exerts greater attraction for the toner than does the carrier and retains the toner in the charged areas and separates it from the carrier particles. The carrier particles, being oppositely charged and having greater momentum, will not be retained by the charged areas of the plate. When a toned carrier particle passes over a non-charged area of the plate, the electrostatic attraction of the carrier particles for the toner particles is suificient to retain the toner on the carrier preventing deposition in such areas as the carrier particles momentum carries both toner and carrier past. By this means the image is developed, i.e., made visible.

This process, known as cascade carrier development, has a high development latitude and is particularly noteworthy in freedom from background deposition. Further, the process is dependable, operates with high efliciency under extreme humidity conditions and is easily converted to give either positive or reverse reproduction of the original to be copied. The process also has certain limitations. Thus, the regular cascade carrier development gives no solid area coverage, that is, solid colored areas such as those presented by block letters develop only around the periphery leaving a white or undeveloped area in the center. Again, relying largely on gravity to move the carrier across the image-bearing surface, the process requires relatively large carrier particle sizes for best efficiency; and, finally, there is a certain tendency for the smaller carrier particles to be retained on the plate thereby interfering with transfer of the toner image. Despite these difliculties, cascade carrier development is the most widely used development process in commercial .use.

As disclosed in US. 2,618,551 to L. E. Walkup, carrier particles such as are used in the cascade carrier process may have magnetic properties. When a developer mix comprising a toner and a ferromagnetic carrier material is contacted with a magnet, streamers are formed which constitute a brush-like mass. The brush may then be passed over a surface bearing an electrostatic image whereby the brush contacts the image-bearing surface. The developer is both triboelectrically charged and deposits on the electrostatic image. It is evident that this process, termed magnetic carrier development is similar to the cascade carrier development. The processes differ mainly in that magnetic field producing means are substituted for gravity as the force controlling the movement of the carrier particles over the image-bearing surface.

Magnetic carrier development gives good coverage of solid areas and is eminently suitable for machine application by reason of the greater compactness of the developer system and freedom from dependence on gravity which limits the placement of a cascade carrier system around a rotary drum. Against these advantages, magnetic carrier development is inherently less efficient than cascade carrier development. In magnetic carrier development only part of the brush contacts the image-bearing surface. In addition, the magnetic field restricts the motion of the carrier particles interferring with the individual carrier particles smoothly rolling across the image surface. As one consequence of this, a higher concentration of toner is generally essential in magnetic carrier development. By reason of this and the electrical characteristics which result in solid area coverage, the process gives a high background deposition and is generally characterized by poor development latitude.

An object of the present invention is to provide a novel means for applying electroscopic developer powder to an electrostatic image-bearing member combining the advantages of both cascade and magnetic carrier development while avoiding the limitations previously associated with each.

Another object of this invention is to define a process for developing xe-rographic images by cascade using fine ferromagnetic carrier particles to achieve improved sensitometry and resolution.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the drawings in which:

FIG. 1 is a diagrammatic side cross-section of developing mechanism according to the instant invention.

FIG. 2 is a view in perspective of a modified embodiment of apparatus according to the instant invention.

FIG. 3 is a side elevation partly schematic, illustrating apparatus for practicing the overall xerographic process including the instant invention.

FIG. 4 is a side elevation partly schematic, illustrating apparatus constructed in accordance with the present invention.

FIG. 5 is a side elevation partly schematic of one embodiment of a device constructed in accordance with this invention; and

FIG. 6 is a schematic illustration of another embodiment of the instant invention.

FIG. 7 is a schematic illustration of another embodiment of the instant invention.

For a better understanding of the invention, reference is now had to FIG. 1 as illustrative of the basic process of the instant invention wherein is shown a cross section of a xerographic plate 10 comprising a layer of photoconductive insulating material 11 coated on an electrically conductive backing material 12. An electrostatic image is formed on the xerographic plate in the conventional manner as described above and made visible using cascade carrier development, comprising a toner 13 and a carrier 14. In the instant case the carrier 14 comprises a ferromagnetic material. As a result there is achieved the structure illustrated in FIG. 1 comprising the finelydivided toner particles 13 adhering electronically to the electrostatic image areas 15 and carrier particles 14 associated therewith. The carrier particles 14 comprise either isolated carrier particles remaining on the image-bearing surface 11 after carrier development or the carrier toner mixture may be merely loosely applied to the surface of layer 11 without the application of gravity to remove the carrier particles therefrom as will be described for example, in more detail in FIG. 7. Accordingly, to remove the ferromagnetic carrier particles 14 from the imagebearing surface 11 without deleteriously affecting the toner image thereon, magnetic field producing means 21 is moved across the surface 11 and closely spaced therefrom whereby the lines of force from the magnetic field producing means 21 attract ferromagnetic carrier particles 14 thereto without affecting toner image 13. The resulting scavenging or selective cleaning of surface 11 results in a toner image marked by the absence of larger carrier particles. The smooth, even toner layer is thus enabled to more intimately contact the transfer member and thereby achieve more uniform and efiicient transfer. Further, the removal of the hard carrier particles prior to transfer and cleaning of the plate reduces the abrasion and wear of the plate surface. At the same time, the use of the ferromagnetic carrier particles improves the solid area coverage of the carrier development process and affords increased flexibility of machine design as will be seen below.

FIG. 2 illustrates an apparatus according to one embodiment of the instant invention. As there shown, magnetic field producing means, as a permanent magnet 21, is positioned on guide means 22 positioning magnetic field producing means 21 closely and evenly spaced above the electrostatic image-bearing surface of xerographic plate 10. The magnetic field producing means 21 are then moved across the surface of image-bearing member 10 after development of the electrostatic image thereon using a ferromagnetic carrier as is described above so scavenge ferromagnetic carrier without affecting the toner image.

FIG. 3 shows suitable apparatus for continuously reproducing an image pattern of light and shadow using xerography. The apparatus comprises a cylindrical xerographic plate 10 having a photoconductive insulating surface 11 coated on an electrically conductive surface 12. The cylindrical drum 10 is mounted to rotate on its axis 16. Positioned around the periphery of drum 10 in the direction of motion of the drum are charging means 30, exposure means 40, development means 50, scavenging means 20, transfer means 60 and cleaning means 70.

In operation the drum 10 is caused to revolve whereby a given portion of the insulating surface 11, first passes under suitable charging means 30 followed by exposure to a pattern of light and shadow to be reproduced at exposure station 40. Any suitable charging means known to those skilled in the art may be used, such as corona charging means as described in U.S. 2,588,699 to C. F. Carlson or in U.S. 2,777,957 to L. E. Walkup, brush charging as in U.S. 2,774,921, etc.

After leaving exposure station 40, the portion of the photoconductive surface 11, now bearing an electrostatic image, passes through development station whereby the electrostatic image is made visible by the deposition thereon of electroscopic powder particles in conformity with the electrostatic charge pattern. The development process is the conventional cascade carrier development system as illustrated in U.S. 2,576,047 to Schaffert and is described in U.S. 2,618,551 to L. E. Walkup and in U.S. 2,618,552 to E. N. Wise wherein carrier particles are cascaded over the surface by the combined effect of gravity and inertia. However, it has b en found that some carrier particles are retained on the image-bearing surface 11. Accordingly in the instant process the carrier is ferromagnetic so that after leaving development station 50, the portion of the photoconductive surface 11 bearing the powder image passes through scavenging station 20 comprising magnetic field producing means closely spaced from surface 11. Any ferromagnetic carrier particles retained on surface 11 are removed by the magnetic lines of force emanating from the scavenging means 20.

The portion of surface 11 now bearing the uniform powder image free of carrier particles passes through transfer station 60. The preferred means of transfer is electrostatic transfer in which an electrically insulating member is contacted with the power bearing surface and an electrostatic field applied therethrough of a correct polarity relative to the charge on the powder particles whereby the powder particles are attracted to the image support member. The process is described in more detail in U.S. 2,576,047 to R. M. Schaifert. By reason of the close uniform contact obtained between the image transfer member and the powder image due to the absence of gross irregularities such as those caused by the presence of carrier particles, the highly efficient and uniform transfer is obtained to the image transfer member. Suitable support materials are paper, plastic or the like. The resulting image on the image support member may then be permanently affixed as by heat, vapor, etc.

The surface 11 is then cleaned of any residual powder image, as by the use of a rapidly rotating fur brush, and then recycled in the xerographic process.

The photoconductive insulating layer 11 may consist either of a continuous film of a photoconductive insulating material such as amorphous selenium, sulphur, anthracene, mixtures thereof either with each other or with various additional materials such as tellurium, arsenic, etc. Alternatively, the photoconductive material may be placed on the support in the form of finely-divided particles in a binder composed of a highly insulating resinous material as described in U.S. 2,663,636 by A. E. Middleton. Suitable photoconductors for application in binder form include, but are not limited to the above named photoconductors and also photoconductive phosphors such as the oxides, sulfides and selenides, of zinc and cadmium, mixtures thereof with each other, titanium, dioxide, tetragonal lead monoxide, mercuric sulfide, etc. Suitable insulating binders include silicone resins, acrylic resins, vinyl resins, polyesters and epoxy resins. The photoconductive insulator, either in the continuous film or in a binder, is

coated on a conductive surface 12 such as aluminum, brass, conductively coated glass or plastic, etc. Alternatively, the photoconductive insulating surface 11 rather than being coated directly on the conductive backing 12, as in FIGS. 1 and 3, may be applied to a web or sheet as of paper, plastic or the like which is supplied from a feed roll to a takeup roll so positioned as to move the paper through the charging, exposing, developing, scavenging and fixing stations. The method of operation is the same as that described in FIG. 3 except that here the powder image need not be transferred from the photoconductive surface but instead may be aflixed thereto as by heat, solvent vapors or the like.

An embodiment of a scavenging electrode according to the instant invention particularly suitable for use in automatic machines is shown in FIG. 4 wherein a cylinderical magnetic field producing means 21 rotatably mounted on its axis 26 revolves inside of nonmagnetic shield 23 as of brass, aluminum, or rigid plastic such as phenolfor-maldehyde resin or a glass laminate, etc. The cylindrical field producing means 21 may be a commercially available cylindrical magnet such as those made by the Indiana Steel Company or may be formed by mounting a series of bar magnets around the periphery of a non-magnetic drum. Non-magnetic shield 23 is teardrop shaped and so oriented so that the large end of shield 23 is in close proximity to surface 11 bearing the powder image. The shield 23 is so positioned relative to magnetic field producing means 21 that the lines of force extend through the shield 23 except at the small end of the teardrop. A trough or similar reservoir 24 collects the ferromagnetic carrier particles scavenged by magnetic field producing means 21. The method of operation of the device is as follows:

Suitable means, not shown, as a motor, cause magnetic field producing means 21 to rotate on its longitudinal axis 26. An image-bearing surface 11 passes adjacent to shield 23, the lines of force from magnetic field producing means 21 extend through nonmagnetic shield 23 and through image-bearing surface 11. Any ferromagnetic particles adhering to surface 11 are attracted by the lines of magnetic force to the surface of shield 23. The motion of the ferromagnetic particles over shield 23 due to field producing means 21 rotating on its axis 26 causes the particles to approach the small end of the teardrop shaped shield which is so positioned and adapted that the combined forces of gravity and inertia cause the carrier particles to move along said shield 23 beyond the effective action of the lines of force from said field producing means 21 whereby the particles are desirably collected in container 24.

In FIG. 5 there is illustrated another embodiment of the instant invention wherein the photoconductive insulating material is in the form of a flexible belt or web. As there shown the image-bearing member comprising a layer of photoconductive insulating material 11 coated on an electrically conductive backing 12 electrically grounded as shown, it positioned about driving rollers 25 positioned in a suitable frame, not shown, which carries and drives the continuous web 10 and is constructed of such a size and shape that the flexible web easily bends around and is carried by such rollers without cracking or distortion of the light sensitive coating.

At one end of the web and slightly above the sensitive coating or layer, there is positioned electric charging apparatus 30, such as a corona discharge electrode, connected to a suitable source of electrical potential, not shown, for distributing an electric charge over the surface of the sensitive layer. Original or film strip material 27 from supply reel 28 and collected on driver or takeup reel 29 is led between rollers 31 which hold it substantially firmly in close register with the charged web while light from a source 32 above the strip 27 and web 10 is focused onto the film causing portions of the sensitive layer structure by light passing through the transparent section of the strip to become electrically conductive thereby discharging the electrical charges residing thereon and leaving the remainder as an electrostatic image of the original.

At development station 50 the electrostatic image is developed through the deposition of a powder mixture comprising electroscopic toner particles coated on a ferromagnetic carrier material cascading across the plate. The toner particles deposit in accordance with the charged pattern created at the exposure station.

The developed web is next fed through scavenging station 20 wherein residual carrier particles adhering to the plate surface are removed. Scavenging station 20 comprises a cylindrical field producing means 21 similar to that shown in FIG. 4, an idler roller 33 and a flexible nonmagnetic belt as of thin, flexible brass 23. Idler roller 33 has a smaller diameter than magnetic cylinder 21 and is so mounted that the top surface of flexible nonmagnetic belt 23 slopes down in traveling from magnetic cylinder 21 to roller 33. Idler roller 33 is further positioned so that carrier particles scavenged by the action of magnetic field producing means 21 are returned to the development system 50. The developed plate having a smooth, even powder image thereon is next fed to a projection station whereat a light source 36 is fed through condenser 37 to the surface 11 of plate 10. The surface 11 comprises a specular reflecting surface as of vacuum evaporated vitreous selenium and the image material deposited at development station 50 comprises a light scattering or diffusing material thus resulting in a reflected image reaching lens 38 which is then projected to image receiving surface 39. Plate 10 continues to a transfer station 60 whereat transfer material 61 is fed from supply roll 62 around roller 63 into contact with the surface 11 of plate 10 to takeup roll 64. Roller 63 is desirably of electrically conductive rubber and acts to contact transfer material, 61 with the plate surface 11. While in contact, an A.C. charge as of about 1,000 volts is applied to the back of transfer member 61 by roller 63 which may be connected for example, to an AC. voltage source 65. After the developed image is transferred to web 61, web 61 moves beneath fuser 66 whereat the transferred powder image is permanently aflixed to the surface of web 61. Web 61 is then fed to takeup roll 64.

Optionally, the entire transfer station may be placed into an inactive position or omitted if no use other than projection of the image onto screen 39 is desired. Such an embodiment is illustrated in FIG. 6. In this embodiment, the toner and carrier are both ferromagnetic. In this case scavenging station 20 is positioned after image projection system 35. The magnetic lines of force from magnetic field producing means 21 remove both residual carrier and the major portion of the toner particles which are collected in suitable receiving means as a trough 24. As the web is recycled through brush-cleaning station 70, only the residual toner remaining on the web is cleaned. The reduction in the amount of toner removed by the brush reduces abrasion of the plate surface and the dust associated with operation of the device. Further, the toner scavenged and collected in trough 24 may be reused in replenishing development means 50.

Several variations of this embodiment may be used. Thus, in FIG. 6 exposure means 40 is a cathode ray tube. If desired however, nonoptical means may be used to form the electrostatic image. Such a device is described in an application for letters patent S.N. 748,655, filed by L. E. Walkup on July 15, 1958. The device illustrated, termed a pin tube, converts an electrical signal such as a facsimile transmission into an electrical charge pattern on an electrically insulating member denotative of the intelligence transmitted. Being nonoptical, layer 11 need be only electrically insulating. A preferred material is polyethylene terephalate. The specular surface may be obtained by vacuum evaporating a thin layer of aluminum on the reverse side of layer 11 so that layer 12 acts as the reference electrode for image transfer and as the reflecting surface at the image projecting station. In this embodiment charging device 30 may be omitted, or, desirably, an unbiased AC corona generating means substituted therefor to neutralize any charges on surface 11 as from triboelectric contact with brush 70. Further, being nonoptical the entire operation may be carried out in room light.

FIG. 7 shown still another embodiment of the invention wherein the toner-carrier combination is dusted as from hopper 50 onto the surface of web with the electrostatic image thereon. In the embodiment shown in FIG. 7, no means are provided for conventional xerographic cascade whereby gravity removes the bulk of the carrier from contact with the image-bearing surface. However, by using a ferromagnetic carrier and a nonferromagnetic toner, magnetic scavenging means 21 effectively removes all carrier from the image-bearing surface without disturbing the toner abstracted from the electrostatic image on said surface and returns the carrier and unexpended toner clinging thereto to development station 50. Thus, whereas conventionally cascade development is restricted to a vertical portion of the image bearing surface, by means of magnetic scavenging it is possible to use cascade development on the upper horizontal image hearing surface.

In conventional cascade, it is preferred to use a relatively coarse carrier with a particle of about to 50 mesh. These carrier sizes when used in cascade development tumble over the xerographic plate surface in a manner that produces deterioration of the plate from the impacts of repeated impringement. This is a major factor in limiting the usable lifetime of these plates. The size and mass of these carriers also reduce the quality of the reproduction from a sensitometry aspect. That is, small voltage gradients in the latent electrostatic image do not get developed since the large carrier particles have both a strong holding power with respect to the toner and a strong dislodging effect on deposited toner that is loosely held. In order to overcome the impact damage produced by the large carrier particles and also to improve sensitometry, attempts were made to use smaller carrier particles. Two major difiiculties were encountered. One, because of the lower momentum too many carrier particles adhered to the plate obstructing toner deposition and, two, residual carrier particles adhering to the plate interfered with image transfer.

Other experiments have shown that metallic or low resistivity carrier material improves solid area coverage. This apparently is because a fairly solid mass of low resistivity carrier particles over an area of a xerographic plate have the effect of a development electrode. Such a mass appears to the latent electrostatic image as an equipotential plane and some of the electrostatic lines of force from the image extend outward toward the mass of carrier particles rather than inward to the conductive plate backing. Thus carrier particles that have low resistivity for Xerographic purposes particularly having resistance less than 10 ohm-centimeters enable improved continuous tone and solid area reproduction in cascade development. Two factors have been found significant in using low resistivity carriers for solid area reproduction. The first is the effective proximity of the carrier to the plate surface. This is inversely related to carrier size. The largest part of each particle is the distance of the particle radius from the plate surface. Reducing the particle radius and therefor the particle size brings the effective proximity of the particle closer to the plate. The closer the particles are to the plate surface, the more readily the electrostatic lines of force extend outward to the carrier particles. The carrier particles have to be cascaded in a single large mass covering a substantial image area. Experiment with cascading has shown that as carrier particles exceed 100 microns in diameter they have a successively greater tend ency to break away from the mass and spread out. In the usual cascade operation this results in good solid area cov- 8 erage at the top of the cascade zone with substantially inferior coverage toward the bottom of the zone. Thus more uniform coverage is obtained by using small carrier particles which tend to cascade uniformly in a solid mass rather than running away from each other.

In a series of experiments using non-magnetic toner and ferromagnetic carrier particles with the carrier particles in size ranges extending up through 350 microns, not only were the anticipated results borne out, but several other interesting features appeared. First of all, it was discovered that in cascading with carrier particles below about 44 microns the tendency of the carrier particles themselves to cling to latent image areas interfered excessively with the deposition of toner. Thus, with these very small carrier particles, even when the carrier particles were removed by magnetic scavenging cleanup, the transferred image was very poor and had serious deficiencies in image areas where carrier particles had blocked deposition of toner. As the size of the carrier particles was increased to 44 microns and above this problem became negligible and the benefits of using small low resistance carriers became obvious. Higher resolution was obtained than when using conventional cascade developer under identical conditions. Greatly improved solid area coverage and gray scale range also became apparent in the transferred image as expected. But, of particular interest was a spectacular increase in maximum density in relatively solid areas to a level which had previously been attained only in line copy. The deepest blacks approached the density of solid toner in solid area diameters well over one quarter inch. While some speculation has risen as to the causes behind the greatly improved solid area density, one of the factors is quite probably that with smaller carrier particles there is a larger carrier surface area available for a given mass of carrier enabling the use of a higher percentage of toner. The value of this becomes more apparent when it is considered that the proportion of toner that can be added to the developer mix is strictly limited by the carrier surface area available. Toner in excess of this amount produces considerable background and nonuniformity in development since it lies around loosly nonadherent to carrier particles. It is believed that there are also other factors involved in this density improvement that are not fully understood at this time.

As the size range of carrier was increased, in the process of experimentation, it was found that as the size of the carrier exceeded about 250 microns the density characteristics slowly deteriorated. At some point between 250 and 297 microns the benefits of using the small size carrier and magnetic scavenging are lost. Improved solid area coverage from the use of low resistance carrier continues to a certain extent but density falls off, resolution starts dropping and a non uniform mottled appearance becomes evident. Further, toward the top of this size range, the mass of low resistance metallic carrier particles used become great enough so that in the cascade process the inertia of the particles was adequate to overcome the need of magnetic scavenging cleanup.

In the course of experimentation it became further apparent that the inventive process is Well suited to reversal development in which the uncharged areas of a latent electrostatic image are developed. As is known in the art, this is accomplished by selecting the correct triboelectric relationship between the toner and the carrier or a coating on the carrier so that the toner becomes charged to the same polarity as the charged areas of the latent image.

Thus, surprising and advantageous results have been achieved using small low resistance ferro magnetic carrier particles in a size range of about 44 to 275 microns in diameter in combination with magnetic scavenging cleanup. A preferred carrier size range is 44 to 210 microns. This preferred size range has nearly optimum resolution and density in accordance with the invention 9 as well as good development electrode effect from minimal scattering of the carrier particles during cascade.

In conventional magnetic carrier development, the carrier particles are oriented by the magnet so that they are not completely free to roll over the image surface. As a consequence of the less complete contact, toner concentration is more critical in magnetic carrier development than in cascade. In magnetic scavenging, however, toner concentration may be varied as widely as in cascade carrier development. In addition, the ferromagnetic carrier particles significant-1y improve solid area coverage.

Any ferromagnetic material may be used as a carrier. Thus suitable materials include ferromagnetic ferrites (as described in US. 2,846,333 by J. C. Wilson), powdered iron such as the types known commercially as alcoholized iron and carboxyl iron, etc. Where the ferromagnetic material does not have the desired triboelectric relationship to the toner, the ferromagnetic material may be used as a core and covered with a resinous coating having the desired triboelectric properties as described in US. 2,618,551 by L. E. Walkup.

Any of the nonmagnetic toners known to those skilled in the art may be used. Suitable toners are described in US. 2,618,551 by L. E. Walkup, US. 2,618,552 by E. N. Wise, US. 2,638,416 by Wal kup and Wise, US. 2,753,308 by R. B. Landrigan, US 2,788,288 by Rheinfrank and I ones etc.

There has thus been provided an improved novel process and means for applying electroscopic developer powder to an electrostatic image. While the present invention, as to its objectives and advantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A cascade development process for Xerographic images comprising:

(a) gravity-cascading a two-component developer consisting essentially of ferromagnetic carrier particles having diameters in the range of 44 to 275 microns and electroscopic nonmagnetic resinous toner particles across a surface carrying a latent electrostatic image to deposit a predominance of toner particles and some carrier particles in the image areas; and,

(b) sweeping said surface with a closely spaced magnetic device to effect the removal of residual carrier particles from all areas of said surface including said developed image.

2. A method of Xerographically reproducing an original with high density solid areas comprising:

(a) forming a latent electrostatic image of said original, on a supporting surface;

(b) gravity cascading carrier-type developer consisting essentially of ferromagnetic carrier particles with large dimension diameters in the range of 44 to 275 microns and resinous nonmagnetic toner particles across said latent electrostatic image to produce a developed image of predominantly toner particles and some carrier particles in the image areas;

(c) magnetically removing residual carrier particles from all areas of said surface including the developed image and (d) transferring said developed image substantially devoid of said carrier particles to a transfer sheet.

3. A Xerographi reversal development process providing high density coverage of solid areas that are relatively wide in comparison to line copy comprising:

(a) gravity-cascading carrier-toner developer composi tion, in which the carrier consists essentially of 44 to 210 micron ferromagnetic particles and the toner consists essentially of relatively fine nonmagnetic colored resinous particles of predetermined triboelectric polarity with respect to said carrier, across a surface carrying a latent electrostatic image in which the charge retaining areas are of the same polarity as said predetermined triboelectric polarity to deposit a predominance of toner particles and some carrier particles in the image areas; and,

(b) selectively removing residual carrier particles from all areas of said surface including said developed image by magnetic scavenging means to leave a readily transferrable toner image substantially devoid of said carrier particles.

4. A xerographic development process for utilizing the advantages of small carrier particles in cascade development comprising:

(a) making a developer composition consisting essentially of ferromagnetic carrier particles in a size range of 44 to 210 microns diameter and enough triboelectrically related resinous nonmagnetic toner particles to substantially cover the surface area of said carrier particles;

(b) gravity-cascading said developer composition over a latent electrostatic image forming a developed image of predominantly toner particles and some carrier particles in the image areas; and,

(c) magnetically removing any of said carrier particles remaining in the area of said developed image without disturbing the toner particles to form a developed image thereon substantially devoid of said carrier particles.

5. A method of printing which includes the steps of producing a latent electrostatic image on an electrically insulating layer, developing said electrostatic image by flowing under the force of gravity to said insulating layer granular ferromagnetic carrier particles in a size range of about 44 to 210 microns diameter having coated thereon releasable resinous nonmagnetic electroscopic toner particles whereby a predominance of toner particles and some carrier particles deposit on said electrically insulating layer in conformity with said electrostatic image forming a developed image corresponding thereto and after said developed image is formed activating magnetic field producing means adjacent said insulating layer so as to differentially remove the ferromagnetic carrier particles from all areas of said insulating layer including said developed image toward said field producing means thereby removing said carrier particles from said insulating layer without disturbing said image to form a developed image substantially devoid of said carrier particles.

6. A method according to claim 5 wherein said electrically insulating layer is composed of a photoconductive insulating material.

7. A method of printing which includes the steps of producing a latent electrostatic image on an electrically insulating layer, developing said electrostatic image by cascading over said insulating layer under the force of gravity granular ferromagnetic carrier particles in a size range of 44 to 275 microns diameter having coated thereon releasable resinous nonmagnetic electroscopic toner particles whereby a predominance of said toner particles and some carrier particles deposit onsaid electrically insulating layer in conformity with said electrostatic image forming a developed image corresponding thereto and after said developed image is formed activating magnetic field producing means positioned adjacent said insulating layer so as to remove the ferromagnetic carrier particles from all areas of said insulating layer including said developed image toward said field producing means without disturbing said image to form a developed image substantially devoid of said carrier particles.

8. A method according to claim 7 wherein said electrically insulating layer is composed of a photoconductive insulating material.

9. The method of printing according to claim 7 in which said ferromagnetic carrier particles have electrical resistivities of less than 10 ohm-centimeters.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Osborne 11717.5X Walkup 11717.5 X Wise 117-17.5

Wilson 117-17.5 Grieg 11717.5 Olden 117--17.5 X

12 2,911,330 11/1959 Clark 1341 2,914,403 11/1959 Sugarman 117-17.5 X 2,970,299 1/1961 Epstein et a1 117-l7.5 X 3,093,039 6/1963 Rheinfrank 11717.5 X 5 WILLIAM D. MARTIN, Primary Examiner.

G. L. HUBBARD, M. SOFOCLEOUS,

Assistant Examiners. 

1. A CASCADE DEVELOPMENT PROCESS FOR XEROGRAPHIC IMAGES COMPRISING: (A) GRAVITU-CASCADING A TWO-COMPONENT DEVELOPER CONSISTING ESSENTIALLY OF FERROMAGNETIC CARRIER PARTICLES HAVING DIAMETERS IN THE RANGE OF 44 TO 275 MICRONS AND ELECTROSPIC NONMAGNETIC RESINOUS TONER PARTICLES ACROSS AND SURFACE CARRYING A LATENT ELECTROSTATIC IMAGE TO DEPOSIT A PREDOMINANCE OF TONER PARTICLES AND SAME CARRIER PARTICLES IN THE IMAGE AREAS; AND, (B) SWEEPING SAID SURFACE WITH A CLOSELY SPACED MAGNETIC DEVICE TO EFFECT THE REMOVAL OF RESIDUAL CARRIER PARTICLES FROM ALL AREAS OF SAID SURFACE INCLUDING SAID DEVELOPED IMAGE. 